The invention relates generally to thermal ink jet printing and, more particularly, to printheads with polysilicon resistive heaters provided with improved resistance control.
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. An ink nucleation process is initiated by individually addressing resistors with short (2-6.mu. second) electrical pulses to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
In a prior art printhead 8 of the type disclosed in U.S. Pat. No. 4,951,063, whose contents are hereby incorporated by reference, and shown in partial cross-section in FIG. 1, a silicon heater substrate 28 has formed on its surface a field oxide layer 39. Polysilicon heater elements 34 are formed followed by a reflowed PSG, thermal oxide composite layer 13, which serves to protect and insulate the heating elements. Layer 13 is masked and etched to produce vias for subsequent interconnection with addressing electrodes 33 and common return electrodes 35. In addition, layer 13 is concurrently removed from the central bubble generating region of the heater element 34. A pyrolytic silicon nitride layer 17 is deposited directly over the heater elements. Layer 17 has a thickness of between 500 .ANG. to 2500 .ANG. and, optimally, about 1500 .ANG.. A tantalum layer 12 of 0.1 to 1.0.mu. thickness is deposited on layer 17. Layer 12 protects the heater element 34 from the corrosive effects of the ink and layer 17 provides electrical isolation. For electrode passivation, a silicon dioxide and/or silicon nitride film 16 is deposited over the entire heater surface followed by a thick insulative polymer layer 18.
Ink in fill channels 20 flows into recess 26 overlying the passivated resistor elements. When the resistor element is pulsed, ink is heated and expelled through nozzle 27 in the printhead front face.
A problem with the prior art fabrication of the type of printhead shown in FIG. 1 is that the nitride layer 17 is typically deposited by a low-pressure chemical vapor deposition (LPCVD) process, a process which produces a nitride layer with a high compressive stress of up to 6.times.108 dynes/cm.sup.2. This highly stressed layer applies a mechanical strain to the underlying polysilicon layer 34, resulting in changes in resistivity of the layer due to piezoresistive effects and to redistribution of dopants between the polysilicon grain boundaries and in the crystallite bulk. Since the amount of stress varies between fabrication runs as a function of the total amount of deposition which has been performed in the reactor and with the age and condition of the vacuum system, the increase in the polysilicon resistance also varies making it difficult to fabricate printheads with consistent resistor heater characteristics. The magnitude of this problem increases with increasing heater polysilicon resistance.
Other potential problems with the prior art process are experienced at the "step" areas when the nitride and tantalum layers conform to the slope of the glass oxide composite layer 13. As shown enlarged in FIG. 1, the deposited layers have higher stress at the step edges 40 sometimes causing cracking. The deposited layers also tend to thin out along area 42, which can further encourage cracking. A third potential problem is that the nitride layer 17 could be undercut at areas 44 during the etch process reducing the quality of the seal to layer 13. All three of these mechanisms offer potential leakage paths for the ink to infiltrate the seal over the heater, which results in ink electrochemically attacking the polysilicon resistor element itself and destroying the heater structure, or causing an electrical short circuit and destroying the driver or addressing circuitry.